1Field of the Invention
This invention generally relates to a lawn mower having at least one rotary cutting blade and an internal combustion engine for operating the at least one cutting blade. A typical lawn mower is generally designed for operation on a substantially horizontal surface, while inclination of the mower on various surface slopes is also possible. Thus, in what could be considered to be a normal position of the mower, or a horizontal position, the internal combustion engine would generally be considered to be vertical. In this normal position, the engine could essentially be considered to have a vertical crankshaft, at least one overhead camshaft and a transmission disposed between the crankshaft and the at least one camshaft to operate the crankshaft and camshaft in a predetermined synchronization with respect to one another. The transmission can essentially be considered to define a plane of movement, inside a transmission chamber, and perpendicular to the crankshaft axis.
2. Background Information
In general, a lawn mower, such as a walk behind lawn mower, is powered by what could be considered to be a small drive motor, such as a motor having only a single cylinder. Such small drive motors can also be used for operating various other devices, such as, for example, motorized bicycles (mopeds), portable generators, etc. For such small drive motors, a precise control of the valve timings is needed to activate the intake and exhaust valves. For this purpose, overhead camshafts, i.e. camshafts which are mounted so that they can rotate in the cylinder head, are being used to an increasing extent to directly or indirectly activate the intake and exhaust valves located in the cylinder head. A transmission between the camshaft and the crankshaft provides the required drive energy, and also provides the required synchronization of the rotation of the camshaft with the rotation of the crankshaft.
In many applications of an internal combustion engines, the drive shaft driven by the engine is oriented vertically. As discussed above, one example of such an application is a rotary lawn mower. But in this case, the vertical orientation is not always maintained, and there can be a corresponding inclination of the drive shaft as a function of the slope of the ground. With regard to the simplest possible construction and high efficiency, and to eliminate losses due to friction, a direct coupling of the crankshaft of the drive engine to the drive shaft, with the interposition of a simple spur gear, is of great advantage. The crankshaft then also assumes a vertical orientation, or an orientation more or less oriented toward the vertical, depending on the slope of the ground.
The at least one "overhead" camshaft in such a configuration can extend laterally and axially parallel to the axis of the crankshaft. To guarantee reliable operation, in particular at high operating temperatures and high engine speeds, provision must be made for continuous lubrication of the camshaft.
German Patent No. DE 40 15 610 A1 discloses an internal combustion engine of the type described above, on which there is an oil pump in the vicinity of the bottom bearing point of the crankshaft. The oil pump can thus provide positive lubrication, both of the crankshaft connecting rod bearing which is subjected to a particularly severe load, and also of the top bearing point of the overhead camshaft mounted in the cylinder head. For this purpose, the oil pump is connected to a passage in the crankshaft which has both a discharge hole to the connecting rod bearing, and also, in the vicinity of the upper end of the crankshaft, makes a transition into cooling passages of a hollow disk flywheel connected to the crankshaft. The cooling passages in turn empty into a cooling passage which runs parallel to the cylinder axis, inside the hollow cylinder wall which surrounds the piston. This cooling passage has both a discharge point which empties into the crankshaft chamber, and also a discharge point which leads to the upper camshaft bearing. The oil which is discharged from this bearing can flow downward along the camshaft, thereby also lubricating the cam surfaces, as well as the transmission formed by a belt drive between the camshaft and the crankshaft. In this manner, the bottom camshaft bearing can also receive sufficient lubricating oil. The lubricating oil flowing back is collected in a separate, obviously lower-lying oil pan, and is returned to the oil pump.
The positive lubrication of this internal combustion engine requires a complex and expensive construction.
European Patent EP-0 487 960 A1 discloses an engine with an overhead camshaft and a vertically-oriented crankshaft, in which the transmission, consisting of a belt transmission, lies at the upper end of the crankshaft and camshaft. Such a configuration essentially also requires a forced lubrication system for the upper crankshaft bearing and for the upper camshaft bearing. The oil which is transported to the upper bearings can then drip from the upper bearings to lubricate both the cam surfaces and the lower camshaft bearing, and then flow back to an oil reservoir annulus surrounding the crankshaft and housing the oil pump. Because of the upper position of the transmission, the transmission itself does not come into contact with the lubricating oil.
Finally, U.S. Pat. No. 5,000,126 discloses an internal combustion engine for a lawn mower which does not use an overhead camshaft, but instead uses a camshaft mounted in the crankshaft chamber.